Material that's only visible if seen through another material
Window:



Material seen through window:



Viewport render showing Suzanne through window:



Viewport render showing invisible Suzanne next to window:



What I have is a Glass BSDF window, slightly off white in color so to be seen, and with a 1.00 IOR to be in all rights except one a Transparent BSDF. Glass BSDFs refract rays like glass refracts light, and I can test for that refraction (called a Transmission Ray).

My Suzanne has a material (red Diffuse BSDF, but can be anything) that is mixed with a pure white (clear) Transparent BSDF. The mix Fac:, whaddoyouno, just happens to be a Transmission Ray :)

.blend file: